A method is disclosed for compatible AM stereo transmission, with predistorted in-phase sidebands, so that the predistortion cancels the distortion generated by the envelope detector of a receiver due to the presence of quadrature sidebands.
There are a number of prior-art compatible AM stereo transmitters, transmitting the signal sum in part by in-phase sidebands, and the signal difference in part by quadrature sidebands. These systems have material distortion of the signal sum at the output of an envelope detector, due to the presence of quadrature sidebands. In these systems the power available for the sidebands of each type is not equal.
In the invention, the carrier with predistorted in-phase and quadrature sidebands is delivered at the transmitter to an envelope detector, which delivers a signal combination, say the sum, without distortion due to the quadrature sidebands, and is also delivered to a synchronous detector, which delivers the signal sum with predistortion. The difference of the detector outputs, which is the predistortion, is returned to the input of the signal-sum modulator as negative feedback. The transmitter then emits a wave with the signal-sum sidebands predistorted, and undistorted signal-difference sidebands. At an envelope detector of a receiver, the signal-sum predistortion is substantially cancelled by the detector distortion due to the presence of quadrature sidebands. This transmitter is stable, due to the negative feedback, has equal power available in each channel, and causes no material distortion of the signal in an envelope detector. It has therefore substantial utility over prior-art transmitters. It uses well-known circuit elements and is based on well-known theory.